In modern gas turbine engines working medium gases having temperatures in excess of 2,000.degree. F. are expanded across rows of turbine blading for extraction of power therefrom. A shroud, termed an outer air seal, circumscribes each row of turbine blading to inhibit leakage of working medium gases over the blade tips. The limitation of the leakage of the working medium gases is crucial to the achievement of high efficiencies in such engines. The graded ceramic seals described herein were developed for specific application in gas turbine outer air seals, although other applications are clearly possible. Durable seals capable of long-term, reliable service in the hostile turbine environment were required. Specifically sought were high temperature capability and good resistance to thermal shock. In addition, the seal material must have adequate surface abradability to prevent destructive interference upon occurrence of rubbing contact of the seals by the circumscribed turbine blading.
U.S. Pat. Nos. 3,091,548 to Dillion entitled "High Temperature Coatings"; 3,879,831 to Rigney et al entitled "Nickel Base High Temperature Abradable Material"; 3,911,891 to Dowell entitled "Coating for Metal Surfaces and Method for Application"; 3,918,925 to McComas entitled "Abradable Seal"; 3,975,165 to Elbert et al entitled "Graded Metal-to-Ceramic Structure for High Temperature Abradable Seal Applications and a Method of Producing Same" and 4,109,031 to Marscher entitled "Stress Relief of Metal-Ceramic Gas Turbine Seals" are representative of the known concepts applicable to ceramic faced seals.
As is discussed in some of the above references and in particular detail in U.S. Pat. No. 4,163,071 to Weatherly et al entitled "Method for Forming Hard Wear-Resistant Coatings", the temperature of the metallic substrate to which the ceramic coating is applied may be preheated to control either residual stress or coating density. Generally, such heating has been to a uniform uniform temperature. U.S. Pat. No. 4,481,237 of common assignee with the present application, describes the production of discrete layered turbine seals wherein the seal is produced by plasma spraying discrete layers of essentially fixed composition on a metallic substrate while simultaneously varying the substrate temperature.
Although many of the materials and methods described in the above patents are known to be highly desirable, the structures resulting therefrom have yet to achieve full potential, particularly in hostile environment applications. Significant research into yet improved materials and methods continues.